Redundant path to internet using 3750G

Unanswered Question
Sep 9th, 2008


Please have a look at the attachment before we start.

We were only connected to ISP A all along and running BGP (though no full routes learned) for Tx/Rx'ing traffic for Network C.C.C.C.

Now we have a backup link ISP B to the internet getting directly terminating on the 3750G. Both ISP A & B are 1 Gb links.

Now with this setup, can we achieve full redundacy (ISP A or link or Cisco 7200 router failure) by re routing the traffic via ISP B. We can not learn the full routes on the cisco 3750G due to the memory limitation.

Any help is appreciated.

Thanks in advance,



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spremkumar Tue, 09/09/2008 - 11:58

Hi Subra

You can run ospf between the switch and the router and announce default route from the router using default route originate command and ensure that you have your traffic flow via primary link.

define a floating static route in the switch pointing towards the router with higher admin distance so that the it can be treated as backup link.

do a as path prepend in the BGP neighborship to the secondary ISP to make sure that you have symmetric traffic flow through single isp during normal condition.

Note : This is just a workaround and i dont think its a best practice to do so.


subra4u Wed, 09/10/2008 - 00:55


Thanks for the Input.

Will this solution help me route packets to/from internet when there is a ISP A failure (meaning ISP A cant reach the internet for some reason). If not, what could be possibly done.

Thanks in advance,



satish_zanjurne Wed, 09/10/2008 - 01:17

For reduandant ISP setup, i will recommend to use router like 3845/7200..

Don't use Catalyst 3750G for this kind of setup.

subra4u Wed, 09/10/2008 - 03:05


We are trying to use the existing Hardware. Thanks anyway.




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